An inter-vehicle communication device is mounted on a vehicle. The device transmits information such as a position of the vehicle to other vehicles disposed around the vehicle. Further, the device receives information such as a position of another vehicle from the other vehicle.
For example, JP-A-2000-90395 discloses a device for transmitting information relating to a vehicle with a short period when the vehicle runs at high speed. The device transmits the information with a long period when the vehicle runs at low speed.
Two vehicles approach each other when the two vehicles run on the same road. In this case, when at least one vehicle runs at low speed, the one vehicle, i.e., the slow driving vehicle transmits the information with a short period by the device disclosed in JP-A-2000-90395. Therefore, the other vehicle may receive the information late from the slow driving vehicle. Thus, a driver in the other vehicle may recognize existence of the slow driving vehicle late. If the road has a traffic jam, and vehicles stop or run at very low speed, another vehicle approaches from behind the vehicles, the other vehicle may crash into rearmost one of the vehicles. The rearmost one of the vehicles runs at very low speed, and is arranged on the rearmost of the vehicles.
Further, when a vehicle turns right or left, the vehicle may runs at low speed or stop temporary. In this case, the device in the vehicle transmits the information with a long period. Thus, another vehicle may receive the information late. Thus, another driver in the other vehicle may recognize existence of the vehicle late, the vehicle turning right or left.
The present inventor has studied about the above situation. FIGS. 7A and 7B show a certain situation according to a related art. In FIG. 7A, three vehicles A, B, C stop or run at very low speed. Here, the very low speed means that the vehicle goes at a crawl. The three vehicles A, B, C provide a group of slow driving vehicles. Another vehicle D approaches from behind of the slow driving vehicles A, B, C. If an inter-vehicle communication device in one of the slow driving vehicles A, B, C transmits information to the other vehicle D with a long period, a driver in the other vehicle D may recognize existence of the slow driving vehicles A, B, C late.
In FIG. 7B, vehicles E-G approach the same intersection. A vehicle G stops in the intersection to turn left at the intersection after two vehicles E, F go through the intersection. Alternatively, the vehicle G may enter the intersection at very low speed. If an inter-vehicle communication device in the vehicle G transmits information to the other vehicles E, F with a long period, a driver in the other vehicle E, F may recognize existence of the vehicle G late.